Field of the Invention
The present invention relates to a machining power supply device for an electric discharge machine which machines a workpiece into a desired shape by changing the relative positions of the workpiece and an electrode by a servomotor while applying a voltage to the machining gap between the electrode and the workpiece to cause an electric discharge across the gap.
Description of the Related Art
In electric discharge machining, rough machining and finish machining are used for different purposes. In the rough machining, the energy of a single discharge is increased to do high-speed processing. In the finish machining, the energy of a single discharge is decreased to do high-precision processing. For the finish machining, a “capacitor pulsed power supply” is widely used in which the charged energy in the capacitor is utilized for machining.
Techniques related to a machining power supply device for an electric discharge machine equipped with a capacitor pulsed power supply are disclosed in Japanese Patent Applications Laid-Open Nos. 2007-038400 and 2003-205426. Japanese Patent Application Laid-Open No. 2007-038400 above discloses a method for electric discharge machining wherein a capacitor after being discharged is recharged with a voltage having the same polarity as that of the residual voltage therein, to implement a higher discharge frequency. In this method, the capacitor charging direction is not reversed.
Japanese Patent Application Laid-Open No. 2003-205426 above discloses an electric discharge machining power supply which has a switch (field-effect transistor; FET) for short-circuiting the machining gap, to prevent a large discharge from being caused by an increased amount of charge accumulated in the gap, thereby ensuring that small discharges occur stably. In this electric discharge machining power supply, although the charge is drawn from the machining-gap capacitance, the charged voltage across the charging/discharging capacitor is not drawn. The charging direction is not reversed either.
In a capacitor pulsed power supply, less variation in the charged voltage across the capacitor leads to a more constant workpiece removal amount for each pulse, ensuring more uniform surface roughness. It also leads to less variation in the gap distance of the machining gap where an electric discharge occurs, ensuring more stable machining. Further, in the case of performing electrode feed control based on an average machining-gap voltage, or, so-called voltage servo feed, the above-described variation in voltage will cause disturbance of the average machining-gap voltage, and therefore, less variation in the charged voltage is desirable. With a conventional capacitor pulsed power supply, however, the charged voltage varies widely. An attempt to suppress the variation leads to a reduced machining speed.
FIG. 5 shows an example of a machining power supply device for an electric discharge machine equipped with a capacitor pulsed power supply.
The machining power supply device 10 includes a DC voltage source V (voltage E), semiconductor switching elements S1 and S3, a capacitor C, a machining-gap voltage detecting unit 5 which detects a voltage across a machining gap between an electrode 2 and a workpiece 3, and a control unit 4 which turns on/off the semiconductor switching elements S1 and S3.
FIG. 6 illustrates an operation of the machining power supply device for the electric discharge machine equipped with the capacitor pulsed power supply shown in FIG. 5. First, at the beginning of a cycle, the semiconductor switching element S1 is turned on for a period T1 to connect the DC voltage source V to the capacitor C for charging the capacitor C. The period T1, which is set in advance, is a time sufficient for the capacitor C to be charged.
Next, the semiconductor switching element S1 is turned off and, at the same time, the semiconductor switching element S3 is turned on to connect the charged capacitor C in parallel with the machining gap between the electrode and the workpiece. When the gap between the electrode and the workpiece is sufficiently small, an electric discharge occurs at a certain time point, and the charge accumulated in the capacitor C is discharged to the machining gap, whereby removal processing of the workpiece 3 is carried out.
Here, the machining-gap voltage detecting unit 5 checks for the occurrence of electric discharge, and upon detection of an electric discharge, the semiconductor switching element S3 is kept on for a period T2 from the time point when the discharge was detected. It may be configured such that a machining-gap current Ig is detected by a detecting unit (not shown). Here, the machining-gap voltage is denoted by Vg. The period T2 is a time sufficient for the capacitor C to be fully discharged. At this time, the capacitor C is charged in the direction opposite to the direction in which it was originally charged. This voltage (residual voltage) in the opposite direction varies depending on the state of the machining gap. Thereafter, a quiescent period is provided for a period T3 for recovery of insulation between the electrode and the workpiece. The above completes one cycle.
In the case where the semiconductor switching element S1 is turned on again in the next cycle to charge the capacitor C, the charged voltage jumps to exceed the power supply voltage E, as shown in FIG. 6. The amount of the jump depends on the residual voltage in the opposite direction. That is, the voltage jumps higher with a greater residual voltage (absolute value thereof). The above-described mechanism causes the charged voltage across the capacitor C to vary.
One of the most reliable ways of suppressing the variation in the charged voltage across the capacitor C will be to insert a current limiting resistor R in the charging circuit for the capacitor C (see FIG. 7). Even if the capacitor C is charged in the direction opposite to the direction in which it was originally charged, when the capacitor C is charged in a next cycle, the current limiting resistor R plays a damping role, so the variation in the charged voltage across the capacitor C can be prevented.
This approach, however, increases the time taken for charging the capacitor C, leading to reduction of the machining speed.